The present invention relates to a transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor. In particular, the present invention relates to a transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor and a method, which can be used for integrity monitoring of structures and measuring strain and temperature.
Generally, optical fiber sensors have small size. Therefore, it is relatively easy to attach the sensors on the surface of objects to be measured and insert the sensors into composite material. In addition, the sensors have high durability and anti-corrosive capability and are not affected by electromagnetic wave. Especially, interferometric optical fiber sensors have high resolution and sensitivity and therefore they are quite useful to monitor integrity of machine structures and building structures caused by unexpected impact and longtime usage.
The optical fiber sensors include interferometric sensors, spectrum analysis-based sensors, and intensity-based sensors. Interferometric sensors are cheaper than spectrum analysis-based sensors and it is simpler to build systems with interferometric sensors than with spectrum analysis-based sensors. Interferometric sensors are more sensitive than intensity-based sensors. There are Mach-Zender interferometers, Michelson interferometers, and Fabry-Perot interferometers in the category of interferometric sensors.
Optical fiber sensors based upon Mach-Zender interferometers and Michelson interferometers are easily affected by external disturbances because they includes reference optical fiber. On the contrary, optical fiber sensors based upon Fabry-Perot interferometers provide light with only one optical fiber and therefore they are not significantly affected by external disturbances. Optical fiber sensors based upon Fabry-Perot interferometers are categorized by medium in which interferometry is occurred. If the medium is optical fiber, the optical fiber sensor is called intrinsic. If the medium is air, the optical fiber sensor is called extrinsic. Extrinsic Fabry-Perot Interferometric (EFPI) sensors are easier to fabricate than intrinsic Fabry-Perot Interferometric (IFPI). And extrinsic Fabry-Perot Interferometric (EFPI) sensors have superior mechanical characteristics to intrinsic Fabry-Perot Interferometric (IFPI).
FIG. 1 is a diagram illustrating internal structure of conventional extrinsic Fabry-Perot optical sensor.
Generally, refractive index (n1) of optical fibers is 1.46 and refractive index (n2) of air is about one. Due to the difference of the refractive index (n1) of optical fibers and the refractive index (n2) of air, partial reflection is occurred at the boundary between core of the optical fiber and the air. Following equation 1 shows mathematical equation to calculate power reflection factor and power transmission factor.                                           power            ⁢                          xe2x80x83                        ⁢            reflection            ⁢                          xe2x80x83                        ⁢            factor                    ,                      R            =                                          r                2                            =                                                                    [                                                                                            n                          1                                                -                                                  n                          2                                                                                                                      n                          1                                                +                                                  n                          2                                                                                      ]                                    2                                ≈                                  3                  ⁢                  %                                                                    ⁢                  
                ⁢                              power            ⁢                          xe2x80x83                        ⁢            transmission            ⁢                          xe2x80x83                        ⁢            factor                    ,                      T            =                                                            (                                                            n                      2                                                              n                      1                                                        )                                ⁢                                  t                  2                                            =                                                                    4                    ⁢                                          n                      1                                        ⁢                                          n                      2                                                                                                  (                                                                        n                          1                                                +                                                  n                          2                                                                    )                                        2                                                  ≈                                  97                  ⁢                  %                                                                                        [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
As shown in FIG. 1, Iin, light propagating through single-mode fiber 11 (SMF), is divided into two at the boundary between the single-mode fiber 11 and air. That is, the first reflected light (Iout1), 3% of Iin, is reflected on the first boundary face between the single-mode fiber 11 and air and propagates toward the reverse direction through core of the single-mode fiber 11.97% of Iin is transmitted into air gap, and then propagates back to core the single-mode fiber 11 after reflected on the second boundary between the multimode fiber 12 (MMF) and air. This is the second reflected light (Iout2)
Path difference of 2s (s: gap length) is generated between the first reflected light (Iout1) and the second reflected light (Iout2). Interference caused by the Path difference is observed at optical receiver. The reflection paths except the first reflected light (Iout1) and the second reflected light (Iout2) can be ignored because they are very small.
I=I0{1+cos 2ks}xe2x80x83xe2x80x83[Equation 2]
k: propagation constant, 2 xcfx80/xcex
xcex: wavelength of light source
The optical receiver receives the first reflected light (Iout1) and the second reflected light (Iout2). As the gap length varies, the first reflected light (Iout1) and the second reflected light (Iout2) cause change of phase difference. That is, as the gap length changes, the number of interferometric fringe changes. Therefore, if the number of interferometric fringe is known, variance of the gap length can be known (xcex94s ). At the moment, because resolution is equal to a quarter of wavelength of 10 light source, helium-neon laser (xcex=633 nm) or laser diode (xcex=1300,1550 nm ) are used as a light source for high resolution.
Even though being able to measure changes in single direction, the extrinsic Fabry-Perot interferometric sensors are problematic to measure physical quantity whose direction is varying. To compensate such problem,. Murphy, K. A., Gunther, M. F., Vengsarkar, A. M. and Claus, R. O. proposed a method at xe2x80x9cQuadrature Phase-Shifted Extrinsic Fabry-Perot Optical Fiber Sensorsxe2x80x9d, Optics Letters Vol. 16, No. 4, pp. 273xcx9c275, in 1991. The method includes Quadrature phase-shift EFPI sensors, which employ two extrinsic Fabry-Perot interferometric optical fiber sensors for detecting moving position. However, they are not efficient in that two sets of light source and optical receiver are required and changes of direction are continuously monitored.
In xe2x80x9cMultiple strain state measurements using conventional and absolute optical fiber-based extrinsic Fabry-Perot interferometric strain sensorsxe2x80x9d, Smart Materials and Structures, Vol. 4, pp. 240xcx9c245, 1995, Bhatia, V., Murphy, K. A., Claus, R. O., Jones, M. E., Grace, J, L., Tran, T. A., and Greene, J. A., absolute EFPI, performing absolute measurement through spectrum analysis, is disclosed.
Though the absolute EFPI uses interferometric fringe to perform absolute measurement, it is appropriate only for quasi-static measurement because of scanning time required by spectrum analyzers.
A transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor and a method used for integrity monitoring of structures and measuring strain and temperature are provided.
The transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor includes first single-mode optical fiber and second single-mode optical fiber, laser device, and optical detector.
The first single-mode optical fiber is inserted into an end of a capillary quartz-glass tube and the second single-mode optical fiber is inserted into the other end of the capillary quartz-glass tube. Air gap is formed between the first single-mode optical fiber and the second single-mode optical fiber in the capillary quartz-glass tube. Gap length of the air gap changes in response to magnitude and direction of transformation of the capillary quartz-glass tube.
The laser device launches light into an end of the first single-mode optical fiber. The end of the first single-mode optical fiber is not inserted into the capillary quartz-glass tube.
The optical detector detects interferometric fringe of light. The light is projected from the laser device and passed through the first single-mode optical fiber, the air gap, and the second single-mode optical fiber. The number of occurrence of the interferometric fringe and trend of signal level are determined by change of the gap length.
Desirably, the gap length is decreased and signal level is increased when the capillary quartz-glass tube is compressible-transformed in length direction. The gap length is increased and signal level is decreased when the capillary quartz-glass tube is tensile-transformed in length direction. The signal level is middle value between peak and valley of the interferometric fringe.
Desirably, the first single-mode optical fiber and the second single-mode optical fiber are fixed to the capillary quartz-glass tube by epoxy adhesive.
A method to measure strain with the transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor includes steps of: attaching the transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor to object to be measured; obtaining number of peaks and valleys during transformation of the object, the neighboring peak and valley being half wavelength of the interferometric fringe; determining transformation direction of the capillary quartz-glass tube on the basis of sign of signal-level change, signal level being middle value between peak and valley of the interferometric fringe; and calculating strain by applying resolution and the number of peaks and valleys to following mathematical equation.   ϵ  =            (              λ                  4          ⁢                      L            gauge                              )        ·    n  
xcex/4Lgauge: resolution of optical fiber sensor
n: number of peaks and valleys observed at output signal
Desirably, the step of determining transformation direction of the capillary quartz-glass tube determines that the capillary quartz-glass tube is transformed in compressible direction if sign of signal-level change is positive. The step of determining transformation direction of the capillary quartz-glass tube determines that the capillary quartz-glass tube is transformed in tensile direction if sign of signal-level change is negative.
A method to measure temperature with a transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor includes steps of: attaching the transmission-type extrinsic Fabry-Perot interferometric optical fiber sensor to object to be measured; obtaining number of peaks and valleys during transformation of the object, the neighboring peak and valley being half wavelength of the interferometric fringe; determining if temperature increases or decreases on the basis of sign of signal-level change, signal level being middle value between peak and valley of the interferometric fringe; and calculating temperature change by applying thermal expansion coefficient of the capillary quartz-glass tube, thermal expansion coefficient of the epoxy, thermal expansion coefficient of the object, and the number of peaks and valleys to following mathematical equation.       Δ    ⁢          xe2x80x83        ⁢    T    =            [                        λ                      4            ⁢                          L              gauge                                      ⁢                  (                                    α              H                        -                          α              T                        -                          α              F                                )                    ]        ·    n  
xcex1T: thermal expansion coefficient of the capillary quartz-glass glass tube
xcex1E: thermal expansion coefficient of the epoxy
xcex1H: thermal expansion coefficient of the object
n: number of peaks and valleys